In many localities, a plurality of radio frequency channels are allocated for use by mobile radio transceivers. Each of these radio channels usually includes at least two separate and distinct frequencies, one for transmitting and the other for receiving. Generally, there may be, and usually are many more mobile radio transceivers than there are available radio channels in a given locality.
Consequently, some arrangement is needed for sharing available spectrum space among users. One such arrangement enables a mobile radio transceiver user to obtain access to a radio channel that is not in use (without disturbing the privacy of channels already in use) and to thereafter establish communication with one or more other specifically designated mobile transceivers over that acquired radio frequency channel. When the user is finished using the channel, he or she relinquishes it for use by other mobile transceivers. One such system for establishing communication between mobile radio transceivers through a repeater over a radio frequency channel selected from a plurality of such channels is disclosed in commonly-assigned U.S. Pat. No. 4,360,927 to Bowen et al issued Nov. 23, 1982. The disclosure of this U.S. patent is expressly incorporated herein by reference.
Bowen et al discloses a positive handshake between a mobile radio transceiver originating a call and a repeater operating on a vacant channel. A mobile transceiver desirous of originating a transmission searches a plurality of channels for an idle channel (indicated by the absence of a busy signal or tone which repeaters transmit on channels in use). When the mobile transceiver locates an idle channel, it transmits a busy signal burst which is received by the repeater operating on the idle channel. The repeater, upon detecting the busy signal burst transmitted by the mobile transceiver, transmits an acquisition signal burst which preferably has characteristics which are different from those of the busy signal. The acquisition signal burst is transmitted only if the channel is in fact vacant.
The Bowen et al type mobile transceiver originating the transmission listens for the acquisition signal, and completes a predetermined channel acquisition signalling sequence only after detecting the acquisition signal burst returned from the repeater. If the acquisition signal burst is detected, the mobile station then transmits a call signalling sequence including a first tone followed by a second tone (i.e. a "group signal" or "collect" tone followed by a "sub-group signal") which collectively identify a particular mobile transceiver (or a group of mobile transceivers) with which the user would like to communicate.
The repeater in the Bowen et al system receives and re-transmits the calling sequence to alert other designated mobile transceivers (which if in an "idle" mode are continuously scanning the repeater channels to detect call signalling sequences pre-assigned to them) identified by the transmitted signalling sequence that communication is to be established. The repeater simultaneously transmits a busy signal throughout the channel acquisition call signalling sequence (beginning at the time that a busy signal transmitted by the mobile station is received by the repeater) to ensure that other mobile transceivers searching for an idle channel detect that the channel is in use and therefore do not try to acquire the channel themselves.
When the Bowen et al mobile transceiver has completed transmitting the desired call signalling sequence, it switches back to a receive mode to listen for a busy signal still being transmitted by the repeater. If such a busy signal is still present, the mobile transceiver assumes that the communication channel has been successfully acquired and dedicated to its use, and begins operating in a "ready" mode. The radio transceiver is available for use by a user to communicate voice information over the acquired communication channel via the repeater to another mobile transceiver(s) alerted by the call signalling sequence and "collected" onto the channel. On the other hand, if the mobile transceiver does not detect a busy signal at the conclusion of its call signalling transmission, it resumes scanning the communication channels until another idle channel is found, and attempts to acquire this idle channel in the same manner.
An improved arrangement is disclosed in U.S. application Ser. No. 721,815 filed Apr. 10, 1985. Briefly, this improved arrangement includes additional signalling tones and suppresses transmission of the busy signal during portions of the call signalling sequence to permit a larger number of different groups of mobile transceivers to be served by a single group of repeaters while reducing the possibility of a mobile transceiver "falsing" on voice information present on a scanned communication channel.
Other radio communications systems are disclosed in the following commonly-assigned patents: U.S. Pat. No. 4,347,625 to Williams issued on Aug. 31, 1982 entitled "Arrangement For Cellular Operation Of A Repeater Trunking System"; U.S. Pat. No. 4,409,687 to Berti et al issued on Oct. 11, 1983 entitled "Arrangement And Method For Establishing Radio Communication In A System"; U.S. Pat. No. 4,281,413 to Forrest issued July 28, 1981 entitled "Multichannel Radio Telephone System"; and U.S. Pat. No. 4,376,310 to Stackhouse et al entitled "Mobile Data Terminal Channel Busy Arrangement" issued Mar. 8, 1983.
The following U.S. patents disclose techniques for routing and/or transferring a call from one repeater in a cellular mobile radiotelephone communications system to another depending upon the physical location of a mobile transceiver participating in the call:
______________________________________ U.S. Pat. No. 4,187,398 to Stark (1980); U.S. Pat. No. 4,228,319 to De Jager et al (1980); U.S. Pat. No. 4,398,063 to Hass et al (1983); U.S. Pat. No. 4,475,010 to Huensch et al (1984); U.S. Pat. No. 4,144,496 to Cunningham et al (1979); U.S. Pat. No. 4,435,840 to Kojima et al (1984); U.S. Pat. No. 4,127,744 to Yoshikawa et al (1978); U.S. Pat. No. 4,144,412 to Ito et al (1979); U.S. Pat. No. 4,242,538 to Ito et al (1980); U.S. Pat. No. 4,308,429 to Kai et al (1981); U.S. Pat. No. 3,764,915 to Cox et al (1973); U.S. Pat. No. 3,663,762 to Joel, Jr. (1972); U.S. Pat. No. 4,144,411 to Frenkiel (1979); U.S. Pat. No. 4,384,362 to Leland (1983); and U.S. Pat. No. 3,898,390 to Wells et al (1975). ______________________________________
Although the exemplary arrangements disclosed in U.S. Pat. No. 4,360,927 to Bowen et al and in U.S. application Ser. No. 721,815 are quite successful, further improvements are possible (it should be noted that the discussion herein of these systems refers only to the exemplary embodiments thereof and does not relate to the scope of any of the patent claims in the Bowen et al patent or in U.S. patent application Ser. No. 721,815). In both of these systems, it is assumed that all mobile transceivers which are to participate in communications are in the "idle" mode and are scanning the repeater channels at the time a call signalling sequence directed to them is transmitted over a vacant repeater channel, and that all such mobile transceivers enter the "ready" mode and successfully "lock on" to the newly-dedicated repeater channel in response to the call signalling and remain on this dedicated channel until communications has terminated. A mobile transceiver which "drops off" a dedicated channel or which accidentally or intentionally misses the call signalling sequence which dedicates the channel has no way of later joining the ongoing communications on the now-dedicated channel.
Suppose, for example, that a repeater channel has been acquired by a group of several mobile transceivers (in the manner described previously), and that one of the mobile transceivers in the group temporarily cannot receive the repeater transmissions (e.g., the vehicle that it is on board is driven through a tunnel or behind a hill). The mobile transceiver "drops off" the dedicated channel because, not detecting the transmission of a busy signal (or a carrier) for a prefixed time-out period, it determines that communication on the channel has terminated. The mobile transceiver will thus return to the "idle" mode of operation in which it continuously scans repeater channels listening for call signalling.
Typically, the operator of the mobile transceiver which has "dropped off" the channel will try to rejoin the continuing communications by initiating a call signalling sequence (i.e., he or she keys the microphone to cause the transceiver to enter the "wait" mode and attempt to acquire a repeater channel). If no repeater channels are free, the attempt at acquiring a new channel will be unsuccessful. Even if the mobile transceiver successfully acquires a new repeater channel, communications with the other mobile transceivers in the group will not be established--these other transceivers are still engaged in communications on the original channel, and the calling mobile transceiver has now acquired a different channel. Two repeater channels have been dedicated to the same group of mobile transceivers, and all of the transceivers in the group are nevertheless unable to communicate with one another.
If one of the operators of a mobile transceiver in the same group as the transceiver which "dropped off" the channel realizes that the operator to whom he or she was conversing is no longer present on the channel, the operators of the transceivers still present on the channel may decide to intentionally terminate communications on the dedicated channel and to attempt to acquire a new repeater channel (so that the transceiver which "dropped off" the channel can be "collected" onto a new channel). If, however, the mobile transceiver which had originally "dropped off" the channel has already acquired a new channel, that transceiver will no longer be in the "idle" mode scanning repeater channels waiting for a call and will not detect the new call signalling sequence. These events can occur several times before all of the mobile transceivers in the group are finally collected onto the same repeater channel, causing inefficient use of repeater channels, waste of time, and frustration of mobile transceiver operators.
In many systems, it is desirable to permit mobile transceivers to be included in more than one group of transceivers. For example, the transceiver of a supervisor or a dispatcher may be programmed to enable him or her to call the transceivers of all employees (employee transceivers may be divided into several groups of transceivers), while an employee transceiver may be programmed to permit the employee to contact only a single group of transceivers operated by a subset of fellow employees. Sometimes, therefore, a mobile transceiver included in multiple groups may be busy communicating with transceivers of a first group while a call for a second group (of which the mobile transceiver is also a member) is transmitted by the repeater. Once the communications of the first group have terminated, the mobile transceiver which is also included in the second group may wish to establish communications with the second group of transceivers. Unfortunately, a call signalling sequence directed to the second group of transceivers will not be received by any of them because they are busy participating in communications on a channel already dedicated to the second group.
A similar situation occurs if a mobile transceiver is powered off or is otherwise disabled from receiving repeater transmissions during the time a call signalling sequence directed to a group of which it is a member is transmitted by the repeater. It would be highly desirable to provide some way for a mobile transceiver not participating in communications on a repeater channel dedicated to its own group of mobile transceivers to selectively join the ongoing communications at will.
Systems which use data streams to direct mobile transceivers to channels (see, e.g., the systems disclosed in U.S. Pat. No. 4,352,955 to Kai et al and U.S. Pat. No. 4,127,744 to Yoshikawa et al) have an acceptable solution to this problem despite the limitations imposed by the solution. Any time a mobile transceiver in such systems is not involved in voice communications on a repeater channel, the mobile transceiver monitors a data stream transmitted continuously by the repeater over a dedicated control channel. If a mobile station "drops off" a channel prematurely, the data stream transmitted on the control channel redirects the mobile station back onto the correct repeater channel.
There are serious disadvantages to this data stream approach, however. A mobile transceiver can never intentionally exit a conversation to make or receive another call on a different channel, since the data stream always forces the mobile transceiver to rejoin the conversation. Moreover, in systems in which the data stream is transmitted over a dedicated control channel, the number of channels available for voice communications is decreased by one. One of the many advantages of the systems disclosed in U.S. Pat. No. 4,360,927 to Bowen et al and in application Ser. No. 721,815 is that no dedicated control channel is needed and all available channels can be used for voice communications. It is wasteful to permanently allocate a channel for transmission of the data stream unless there are so great an abundance of voice channels that a channel is always free. Unfortunately, systems installed in busy metropolitan areas can experience overload during times of peak usage. Moreover, it is often not worth the expense of providing a dedicated control channel and associated equipment (e.g. a transmitter) merely to resolve a problem which arises only occasionally when a mobile transceiver "drops off" a channel prematurely.
The present invention permits a mobile transceiver to join, at will, communications occurring on a repeater channel after the repeater channel has been dedicated without requiring a control channel. A group of mobile transceivers scans plural communications channels waiting for a call signalling sequence directed to the group. When such a call signalling sequence is transmitted over one of the channels, the group of transceivers are collected onto the channel and communications are permitted between the transceivers.
When a further transceiver of the group not participating in communications over the channel dedicated to the group is to join (or rejoin) the ongoing communications, the transceiver transmits call signalling over a channel different from the dedicated channel. A repeater operating on this different channel decides, in response to the call signalling transmitted by the further transceiver, whether the further transceiver should join the ongoing communications permitted over the first channel. If the further radio transceiver is to join the ongoing communications, the repeater causes the further transceiver to begin scanning the communications channels and simultaneously, interrupts the ongoing communication and transmits a call signalling sequence over the already-dedicated channel. The scanning mobile transceiver receives the call signalling sequence transmitted over the already-dedicated channel and joins the communications being conducted thereon.
In an exemplary system in accordance with the present invention, an operator of a mobile or portable radio transceiver is free to decide whether or not the transceiver is to remain tuned to a channel dedicated to a group of transceivers of which his or her transceiver is a member. The operator may choose to simply drop off the channel and not participate further in communications thereon, or drop off the channel in order to communicate with a different group of radio transceivers. The operator is free to rejoin the ongoing communications on the channel later by simply initiating a call on an unused channel. Cooperation between repeaters automatically causes the operator's radio transceiver to drop off the channel upon which the call is being initiated, to begin scanning the communications channels, and to be collected onto the repeater channel already dedicated to the group of which the transceiver is a member.
Thus, an operator can intentionally "drop off" a channel without being forced back on. If a mobile transceiver accidentally "drops off" a channel, the operator of the transceiver has the option of deciding whether or not to rejoin the conversation. Although a free repeater channel is necessary for a join to occur, a channel does not have to be permanently allocated for, and dedicated to, implementing the join feature. Since the need for a join does not arise frequently enough to justify allocating a control channel especially for this feature, available resources (i.e., equipment and radio channels) are used more efficiently. Moreover, the call signalling used to cause a mobile transceiver to join an existing conversation serves as an alert to the operators of mobile transceivers already operating on the dedicated channel that one or more transceivers have just been joined and are now also operating on the channel.
The present invention is fully compatible with both the earlier Bowen et al system described in U.S. Pat. No. 4,360,927 and with the improved system described in application Ser. No. 721,815 (although, of course, the invention can be used with advantageous results in other trunking communications systems as well). All changes to existing systems are made in the preferred embodiment by simply reprogramming the repeater, and do not require modification of the many mobile transceivers that may already be out in the field.